When we add 2.0 g of a solute to a solution, the mass of the solution always increases by 2.0 g. If we add 2.0 mL or 2.0 cm3 the volume does not increase by the same amount. Why not? Briefly explain.

My guess is this:

An increase in mass will increase volume because you know how much is being added in grams.

But if you add 2 mL, the volume still increases, but that may not be 2 grams.

Density matters!

Adding 2 mL will only be 2 grams if the density was 1g/mL. Otherwise, it's more or less.